1. Field of the Invention
The present invention relates to an IC (integrated circuit) for an oscillator that is particularly suitable to construct a local oscillator of a TV tuner.
2. Description of the Related Art
A conventional IC for an oscillator (hereinafter referred to simply as “IC”) that is used in TV tuners will be described below with reference to FIG. 3.
An IC 60 is provided with a constant current source 63 and a pair of oscillation transistors 61 and 62 whose emitters are connected to each other. The emitters of the first oscillation transistor 61 and the second oscillation transistor 62 are connected to the constant current source 63. Voltages are applied to the collectors of the oscillation transistors 61 and 62 via power supply resistors, respectively.
The IC 60 is also provided with first to fourth coupling capacitors 64-67.
The IC 60 has a first terminal 60a and a second terminal 60b. The base of the first oscillation transistor 61 and the collector of the second oscillation transistor 62 are coupled to the first terminal 60a via the first coupling capacitor 64 and the second coupling capacitor 65, respectively. The collector of the first oscillation transistor 61 and the base of the second oscillation transistor 62 are coupled to the second terminal 60b via the third coupling capacitor 66 and the fourth coupling capacitor 67, respectively.
The IC 60 is provided with a diode 68 whose cathode is connected to the first terminal 60a and anode is grounded and a diode 69 whose cathode is connected to the second terminal 60b and anode is grounded.
The IC 60 is also provided with a constant current source 73 and a pair of oscillation transistors 71 and 72 whose emitters are connected to each other. The emitters of the third oscillation transistor 71 and the fourth oscillation transistor 72 are connected to the constant current source 73. Voltages are applied to the collectors of the oscillation transistors 71 and 72 via power supply resistors, respectively.
The IC 60 is further provided with a fifth coupling capacitor 74 that is connected to the base of the third oscillation transistor 71, a sixth coupling capacitor 75 that is connected to the collector of the fourth oscillation transistor 72, and a capacitor 76 that grounds the base of the fourth oscillation transistor 72.
The IC 60 has a third terminal 60c. The fifth coupling capacitor 74 connects the base of the third oscillation transistor 71 to the third terminal 60c. The sixth coupling capacitor 75 connects the collector of the fourth oscillation transistor 72 to the third terminal 60c. 
The IC 60 is provided with a diode 77 whose cathode is connected to the third terminal 60c and anode is grounded.
The three diodes 68, 69, and 77 function as protective diodes for preventing the oscillation transistors 61, 62, and 71 from being broken due to static electricity that is introduced externally via the terminals 60a, 60b, and 60c. 
If a resonance circuit is provided between the first terminal 60a and the second terminal 60b of the IC 60, a balanced oscillator is formed that functions with the first and second oscillation transistors 61 and 62. If one end of another resonance circuit is connected to the third terminal 60c and the other end is grounded, an unbalanced oscillator is formed that functions with the third and fourth oscillation transistors 71 and 72.
A first resonance circuit 81 and a second resonance circuit 82 are provided outside the IC 60. The first resonance circuit 81, which is a parallel resonance circuit having an inductance element 81a and a varactor diode 81b, is provided between the first terminal 60a and the second terminal 60b. The anode of the varactor diode 81b is DC-grounded by a resistor 81c, and a tuning voltage Vt is applied to its cathode. This oscillation circuit is used as a local oscillator for receiving a TV signal of the VHF high band.
One end of the second resonance circuit 82, which is a parallel resonance circuit having an inductance element 82a and a varactor diode 82b, is connected to the third terminal 60c and the other end is grounded. The anode of the varactor diode 82b is DC-grounded by the inductance element 82a, and the tuning voltage Vt is applied to its cathode. This oscillator is used as a local oscillator for receiving a low-band TV signal.
The diodes for preventing electrostatic breakdown of the oscillation transistors have equivalent capacitance components (about 1 pF) and are connected in parallel to the resonance circuits. Therefore, in changing the oscillation frequency by means of the tuning voltage Vt that is applied to the varactor diode of the resonance circuit concerned, the capacitance component(s) of the diode(s) remains effective even if the capacitance of the varactor diode is set at a minimum value. This means a problem that the variable ranges of the oscillation frequencies are narrowed.